I'm not sure we're all talking about the same thing.
First, this tool is just a tool. If someone is misusing a tool, don't blame the tool, blame (and block) the user of the tool !
Then it seems that the quality level has not the same meaning on every wikisources. Typo such as « rn » intead of « m » are usually removed on the red or yellow step on fr.ws (and such obvious error can be seen before editing, reviewing the final render code seems enough to me). When I'm thinking of raw code review on yellow to green step, I'm thinking of formatting and things like html code replace by ws templates, Unicode encoding mistakes, and little things like that ; for me all typo should be gone at the previous stage (and personally, I don't go from red to yellow if there is still such typo mistakes).
The GGB is a tool (and just an idea of a tool right now) and one of many solution to one of many problems Andrea pointed ; but there is many other problems. Especially, the navigation arrows could use some improvement. « validate this and go to next page » is definitively something we need. Since the VisualEditor is coming, we would be dumb no to cease this opportunity to do some clean-up and renovation.
We should think too to an other category of tools : global detection of possible mistakes. On frws, there is some little things like https://fr.wikisource.org/wiki/MediaWiki:Gadget-Erreurs-communes.js (intern gadget) and https://tools.wmflabs.org/dicompte/index.php (extern) but here too there is huge room for improvement. Proofreading page by page is great and necessary but we should multiply the approachs to reach the best quality.
We're speaking of new users but such tools (the GGB and much more others) can be useful for old users too. Maybe we can test them for some old user first, see how it goes and then offers them (or not) to new users. Finally, new users are not all the same. The director of Rennes Library is a new user on frws but she's defintively better at proofreading than most wikisorcerers ;)
Cdlt, ~nicolas